1. Technical Field of the Invention
This invention relates to an improved process for reducing the sulfur content in flue gas generated by the combustion of a sulfur-containing fuel. More particularly, the present invention relates to the combined addition of formate and thiosulfate, magnesium, sodium or other calcium concentration reduction agent to improve the performance of and reduce costs in wet, calcium based flue gas desulfurization processes.
2. Description of the Prior Art
Over the past two decades, the consumption of electrical power generated by power generation sources which use sulfur-containing fuels, particularly coal for the generation of steam for producing such electrical power, has increased tremendously while the restrictions placed on the flue gases which emanate from the burning of such sulfur-containing fuels have been tightened nationwide and particularly in areas where a heavy concentration of such gases develop. Therefore, there have been a variety of systems developed for treating the flue gases which emanate from the power generation plant such that the stringent standards promulgated by the Environmental Protection Agency can be met.
These various systems include processes which attempt to reduce or eliminate the sulfur content in the fuel prior to its combustion. Other systems include processes which require the addition of chemical compounds into the combustion zone which change the nature of the sulfur compounds produced, thereby aiding in their removal from the combustion products. Still other systems include scrubbers which require the addition of compounds into the flue gas generated by the combustion of the sulfur-containing fuels to remove the sulfur compounds from the flue gas by way of a chemical reaction.
The after treatment of flue gas may be accomplished by a variety of known processes; namely, wet scrubbing, spray drying or dry treatment of the flue gas in a contact zone of a flue gas flow passage. Illustrative of the wet scrubber approach is U.S. Pat. No. 3,928,537, issued Dec. 23, 1975 to Saitoh et al., which discloses contacting the exhaust gas with an aqueous solution of an organic acid to form a soluble sulfite or sulfate. The sulfite or sulfate is removed, and the organic acid regenerated, by a second step comprising reaction with a calcium compound such as an inorganic or an organic acid salt, for example calcium hydroxide or calcium formate.
The spray dryer approach is illustrated for example by U.S. Pat. No. 4,279,873, issued July 21, 1981 to Felsvang et al., which discloses spraying a suspension of fresh slaked lime and recycled fly ash plus spent calcium compound into the hot flue gas in such a manner as to evaporate the slurry droplets to dryness; the resulting powdered solids are removed from the flue gas by a downstream electrostatic precipitator or bag filter.
U.S. Pat. No. 4,178,349, issued Dec. 11, 1979 to Wienert illustrates the dry treatment; it discloses mixing a dry, powdered lime-bearing material in a reactor, and subsequently separating the solids from the treated flue gas.
Another example of a dry process is the flue gas desulfurization process set forth in U.S. Pat. No. 4,615,871 issued to Yoon which utilizes calcium formate to remove sulfur compounds from combustion exhaust gas in a dry treatment system. In doing so, a finely divided dry solid is formed and subsequently separated from the flue gas. However, systems employing dry injection FGD processes differ from that of wet calcium-based FGD processes in that it is necessary to control the humidity of the flue gas and consequently requires that the environment within the contact zone of the flue gas passage be maintained at a particular optimum level which is both costly and difficult to accomplish. Many additives have been proposed for improving the performance of various aspects of wet calcium based gas desulfurization (FGD) processes. For example, U.S. Pat. No. 4,670,236 issued to Thomas et al. introduces a 50:50 diisobutylene-maleic anhydride copolymer having an average molecular weight of 11,000, However, the introduction of diisobutylenemaleic anhydride does nothing to enhance the desulfurization of the flue gasses and only reduces the formation of calcium scale on the surfaces of the system to reduce maintenance costs.
In U.S Pat. No. 4,454,101 issued to Garrison et al., relatively small amounts of a sodium thiosulfate additive or additives derived from or related to sodium thiosulfate are added into the scrubber liquid slurry. As a result, the thiosulfate ion alters the conglomerative characteristics of the spent slurry crystals making them settle from suspension faster and dewater more readily when filtered. However, the Garrison et al. process is not intended to improve the desulfurization process. Rather, it operates only to improve the dewatering capabilities of sulfite sludges from flue gas desulfurization facilities.
Clearly, there is a pressing need for a flue gas desulfurization process which will improve the efficiency and cost effectiveness of wet, calcium-based FGD systems by reducing the sulfur dioxide content of flue gases being discharged into the atmosphere while minimizing the downtime and replacement costs necessitated by the formation of calcium scale within the system. Also, with the cost of chemical additives increasing, there is a need for a system which will effectively reduce the sulfur dioxide level of the flue gas while consuming a minimal amount of reactive chemical. Further, with the more stringent environmental restrictions which have been implemented by the Environmental Protection Agency, there is a need for a cost-effective and reliable wet, calcium-based flue gas desulfurization process which is capable of meeting current environmental standards.